1. Field of the Invention
The invention relates to terminal antennas for use in telecommunications systems and, more particularly, to an antenna adapted for communication with low earth orbit (LEO) satellite systems.
2. Description of the Related Art
Satellite radio communication has been in existence now for many years. Initially such satellites were designed to route communications from one point on the earth to the other, principally for long distance telephone calls and television signals. For example, the "Early Bird" satellite linked two stations on either side of the Atlantic ocean and enabled the first practical live television coverage of events on opposite sides of the Atlantic. These early satellites were located in geosynchronous orbits and their communication linkages were comparatively low frequency (and thus long wavelength) and required relatively large diameter dish shaped receiving antennas at each earth station.
In general, communication satellites fall in three categories. The first are known as geosynchronous earth orbit (GEO) satellites, which are positioned in orbit at a point approximately 22,000 miles above the earth so they appear to remain "stationary" over the same point on the earth. All the earliest satellites were of this type. The second type of communication satellites are called medium earth orbit (MEO) satellites which are proposed to orbit the earth at a distance of about 8,000 miles. This shorter distance from the earth to the satellite reduces the transmission delay of signals so that real time communication with such satellites is much more practical. For example, a GEO satellite requires approximately 0.25 seconds for a round trip from an earth station to the satellite and back again while an MEO satellite requires less than 0.1 seconds to complete the same circuit. The third type of satellites which are currently being proposed are referred to as low earth orbit (LEO) satellites. These LEO satellites will orbit the earth at a distance of only 500 to 1,000 miles above the earth providing a relatively short distance for the radio signal to travel between an earth terminal and a satellite and thereby reducing the transmission delay to on the order of 0.05 seconds making real time voice and data communications much more practical. In addition, the short distance between earth stations and the satellite reduces the need for sensitive and bulky receiving equipment. Modern satellite constellation systems such as that currently being proposed by a U.S. partnership of companies and referred to as "Teledesic" incorporates such LEO satellites.
The preferred LEO communication satellite constellations are cellular in nature and intended to handle large amounts of data including high speed mobile internet access as well as high speed business data communications. Such large data throughput requires a very large amount of bandwidth in the communication link. In order to obtain this bandwidth these systems will need to operate at relatively high frequencies, for example, in the Ku and Ka bands, and employ frequencies on the order of 12-30 GH.sub.z. It is well known that the higher the frequency of operation of a satellite system the narrower the beam which is available for efficient use by the satellite antenna. Thus, in such systems it is very important to precisely control the receiving/transmitting antenna.
With lower frequency satellite communication, for example in the range of a few gigahertz, a mobile station communicating with a satellite may employ a simple linear antenna structure and communicate with sufficient efficiency that the desired goals are accomplished. However, for very high frequency communications scanning antennas are necessary in each mobile station in order to be able to capture the rapidly moving satellite and achieve efficient communication. The incorporation of such antennas into a mobile station involve a number of technical obstacles.
One characteristic of LEO satellite systems is that due to the apparent movement of each satellite across the sky, the time period during which a mobile station may engage in communication with each particular satellite is relatively short and requires special consideration. For example, a mobile station must be able to establish a communication link with the satellite immediately when it comes over the horizon and is electronically visible to the mobile station and then track that satellite as it passes overhead and disappears over the other horizon. Prior to the disappearance of the "going" satellite, the antenna of the mobile station must be able to establish communication with a "coming" satellite so that there is an effective "handoff" of the communication link from the going satellite to the coming satellite while the communication link with both satellites is still good. One solution to this problem is to provide each mobile station with two antennas. One to track the going satellite across the sky until it disappears and a second antenna to be ready for the appearance of the coming satellite so that there is never any break in the communication link when the handoff from one satellite to the other occurs. Needless to say, multiple antennas for each terminal is both bulky and expensive.
Another solution to the problem of LEO satellite communication is the antenna shown in U.S. Pat. No. 5,650,788 entitled "Terrestrial Antennas for Satellite Communication System" issued Jul. 22, 1997 to Jha and assigned to Teledesic Corporation. This antenna is a hemispherical phased array antenna which is electronically scanned. However, to achieve the high gain which is necessary to handle the large data rates proposed, the antenna must have a large number of phase controlled elements and, thus, be relatively expensive.
Still another solution to the LEO satellite handoff problem would be to provide a very high speed mechanical scanning mechanism on an antenna of fewer elements so that immediately prior to the disappearance of the going satellite, the antenna could be rotated and locked onto the beam of the coming satellite without any interruption in the data stream. However, with very high frequencies and rapidly moving LEO satellites this would require mechanical movements of the antenna system at a speed and precision far in excess of that which mechanical adjustment mechanisms controlled by current technology are capable.
There exists a need for a relatively inexpensive high gain antenna system for a mobile station communicating with LEO satellites which has the capability of compensating for both movements of the mobile station as well as movements of the satellite and of being scanned at a sufficiently high rate to avoid any loss of data communications when being handed off from a going LEO satellite to a coming LEO satellite. The system of the present invention fulfills such needs.